The present invention relates to a process for manufacturing products comprising rigid polyisocyanurate foam.
The manufacture of flexible faced, rigid polyisocyanurate foam insulation boardstock is commonly practiced by a process called restrained rise lamination. The restrained rise process relies on a combination of chemical component blending, precision metering, reactive component mixing and dispensing, use of a moving opposed platen pressure laminator, and use of dimensioning finishing equipment.
In the traditional restrained rise process, isocyanate (xe2x80x9cComponent Axe2x80x9d) is used as received. Component A is supplied by pump to a metering unit, or a metering pump. A premix (xe2x80x9cComponent Bxe2x80x9d) containing polyol, expansion agent, catalyst and surfactant is prepared according to a defined formulation in a mix tank. Component B is also supplied by pump to a metering unit, or a metering pump. The metering pumps boost the pressure generally to 2000 to 2500 psi and control the flow of Components A and B to a precise ratio as determined by the desired chemistry. The pumps deliver Components A and B to at least one foam mixhead. Inside the mixhead, the Components A and B are impinged against each other at high pressure, which results in intimate mixing of the components.
The mixed chemicals exit the mixhead and are dispensed onto a moving bottom facing sheet in a plurality of discrete, liquid streams, in a quantity depending on the type and thickness of desired final boardstock product. The facing sheet carrying the chemical streams then enters a pressure laminator. The spacing, or gap, between the top and bottom platens of the laminator is set to approximately the final desired thickness of boardstock. The laminator temperature is adjusted typically to about 120 to 150xc2x0 F. to insure that no heat is lost from the reacting, exothermic chemical mix, and to insure that the facings adhere well to the rising foam.
The mixed chemicals begin to react in about 5 to 10 seconds following mixing, expanding about 35 to 40 times in volume in the laminator and completing reaction in about 35 to 45 seconds. Laminator speed is adjusted to insure that complete reaction occurs within the pressure section of the laminator. The reaction rate is adjusted by catalyst modification to optimize chemical mixture xe2x80x9cflow.xe2x80x9d Flow is a property of the reacting, rising foam by which expansion is controlled in such a manner that the foam properly expands both upward and sideways to fully fill the moving cavity defined by the laminator. This reactivity adjustment is essential to control both the overall properties of the final product and the cost of manufacture. Improper flow results in poor foam cell geometry which can deteriorate physical, thermal and flammability properties, and causes excessive densification of foam layers in contact with facings.
Rigid boardstock, with facing firmly attached, exits the laminator. This boardstock is trimmed to the desired final width and length. Finished product is conveyed to packaging equipment.
Much of the art in the manufacture of polyisocyanurate takes place where the mixed chemical streams are laid onto the bottom facer prior to entering the laminator. It is necessary that the chemical streams be placed and configured properly to insure that the potential negative effects of the rising foam (e.g., densification of foam at the facer interface through sideways expansion) are minimized. Proper chemical system catalysis is also essential to insure that the rising foam flows properly. Process line speed must be balanced with the foam reactivity so that flow is preserved and the finished boardstock has reached sufficient hardness to be further processed.
When done properly, acceptable foam physical, thermal and flammability properties are achieved. The density spread between core foam density and the in-place density, or IPD, is minimized (core foam density is defined as the measured density of the foam section of one half the thickness of the board taken from the center of the thickness; in-place density is defined as the total quantity of foam chemicals in a complete section of board including layers of surface densification and chemical that has been absorbed into the facers). Typical values for core foam density versus IPD for restrained rise process foam boardstock are 1.75 lb/ft3 for core foam density and 1.95 lb/ft3 for IPD. However, imbalance of laydown, catalyst and line speed can easily drive IPD well over 2.0 lb/ft3.
Typical maximum line speed for a restrained rise process is approximately 1.5 feet/min for each foot of laminator length. That is, a 70 foot long laminator will produce. acceptable quality boardstock at 105 feet/min at minimal cost; a speed of 2.0 feet/min per foot of laminator can be achieved on certain products with catalyst modification and careful attention to operating parameters. It is advantageous to increase line speed, and therefore production capacities, to gain more output from a given piece of equipment.
While mechanical limitations (i.e., finishing saws, conveyors and packaging equipment) can be modified to accommodate higher line speeds by conventional means, maintenance of proper foam properties and cost efficiencies present a more difficult problem. Increased line speed reduces the laminator dwell time (the time that the reacting foam is inside the pressure laminator) and must be altered to complete foam reaction more quickly. As the reaction time is reduce, chemical flow is also altered resulting in a condition commonly known as xe2x80x9clock up.xe2x80x9d When flow is lost, excessive densification at the foam/facer interface occurs, and cell geometry can be altered in a manner such that important properties, including compressive strength, dimensional stability, facer adhesion, insulation value and certain flammability characteristics, are deteriorated. It is therefore advantageous to remove or reduce the need for chemical flow as a component of the process.
Another known process for making flexible faced, rigid polyisocyanurate foam insulation boardstock is the free rise process. In this process, chemical laydown or distribution is accomplished through the use of a pair of matched, precision metering rolls. Chemicals are dispensed just upstream of the metering rolls. The gap between the rolls is adjusted to approximately {fraction (1/35)} to {fraction (1/40)} of the desired finished thickness of the boardstock. This small gap causes the dispensed chemical to form a xe2x80x9cchemical bankxe2x80x9d against the metering roll, forcing the chemical to spread across the full width of the bottom facer. A thin layer of mixed foam chemicals (approximately {fraction (1/35)} to {fraction (1/40)} of the desired finished thickness of the boardstock) is uniformly spread between the top and bottom facers. This composite then moves into a heated oven where the foam reaction is completed. Foam expands 35 to 40 times in volume and becomes sufficiently rigid for further processing. Final foam thickness is controlled by precision adjustment of the metering rolls. No mechanical restraint is utilized for thickness control, as with the restrained-rise process.
The free rise process does not require chemical flow. Dispensed and metered chemicals need only expand in the thickness dimension and not in the width dimension since the original laydown already accomplishes full width application. By removing the need for flow, catalyst adjustments are made only to achieve complete reaction at the desired line speed without the negative impact of xe2x80x9clocking upxe2x80x9d the foam system. The free rise process is capable of speeds in excess of 250 feet/min.
An additional benefit of the free rise process is that density control is achieved within more efficient limits. Since sideways flow of expanding chemical does not occur, densification at the foam/facer interface is minimized. Density spreads of 1.70 lb/ft3 for core foam density and 1.75 lb/ft3 for IPD are routinely achieved.
Although the free rise process presents several significant advantages over the restrained rise process, there are some deficiencies of the free rise process that preclude its use for roof insulation boardstock manufacture. Since the free rise process does not utilize a mechanical means to control product thickness but instead relies on precision metering of chemicals and consistent expansion ratio, thickness variability becomes increasingly exaggerated as overall boardstock thickness is increased, resulting in boardstock that is unacceptable for field application. For example, thickness variation in a 4 inch product can easily be +/xe2x88x920.25 inches, which is unacceptable for many applications. Additionally, typical roof insulation facers are not uniform enough in thickness to provide precision surfaces in the metering roll process. Facer thickness variations will be exaggerated by 35 to 40 times in the final board. Lastly, the free rise process does not employ a mechanical means of foam width formation resulting in excessive waste through edge trim losses. These losses increase as the product thickness increases.
In view of the disadvantages of the prior art processes, there is a need for an improved process for the manufacture of flexible faced, rigid polyisocyanurate foam insulation boardstock.
The present invention is directed to a process for the manufacture of flexible faced, rigid polyisocyanurate foam insulation boardstock which provides excellent boardstock thickness control, minimal wasted densification at the foam/facer interface, improved product properties and high line speed.
Thus, in one aspect, the present invention provides a process for manufacturing an insulation board comprising a rigid polyisocyanurate foam having two major surfaces and a facing material on at least one of the major surfaces, the method comprising:
(a) conveying a facing material along a production line for attachment to one major surface of the foam;
(b) applying a foam-forming mixture of polyisocyanurate to the facing material in a manner comprising spreading the mixture with a spreading means in the direction of the width of the facing material;
(c) optionally conveying a second facing material along the production line for attachment to the other major surface of the foam;
(d) conveying the facing material with applied foam-forming mixture into a laminator which comprises a gap for foam expansion and allowing the mixture to foam and expand to fill the gap within the laminator; and
(e) curing the foam.